The present invention relates to a Radio Transmitter incorporating a Milli-Meter Wave Synthesizer with Frequency-Shift-Keyed (FSK) Modulation and, more particularly, to a FSK Modulated Frequency Synthesizer employing a novel phase-locked loop (PLL) requiring simpler modulation circuitry than that required with prior art FSK PLL Frequency Synthesizers.
The improved FSK PLL Frequency Synthesizer of the present invention incorporates a novel PLL configuration which provides output frequency deviation substantially independent of the frequency synthesizer multiplication factor. This constant frequency deviation versus frequency multiplication factor is due to the new PLL mixing scheme used for frequency multiplication.
In most prior art synthesizers, the input frequency source serves not only as the reference frequency source for the synthesizer, but is also the means for providing frequency modulation of the output frequency. In a prior art PLL, if the output frequency is changed, then the maximum frequency deviation also changes. Thus, in prior art PLL's, if a frequency multiplication of "K" times the reference oscillator frequency would be required, then with a maximum fm frequency deviation of df.sub.max, at a nominal output frequency fo.sub.nom, the resulting actual frequency deviation will be EQU df.sub.act =K*df.sub.max =(fo.sub.act /fo.sub.nom)*df.sub.max.(1)
In contrast, in the present invention, the frequency multiplication is accomplished by a PLL incorporating a novel mixing scheme in which the maximum frequency deviation is substantially independent of the PLL frequency synthesizer frequency multiplication ratio.
Prior art phase-locked loops multiply frequency and maximum frequency deviation proportionately. In the prior art, the frequency multiplication by a given frequency multiplication factor, therefore, has a corresponding proportional frequency deviation multiplication factor. Thus, for a frequency-shift-keyed (FSK) transmitter, or for a general frequency-modulated (FM) transmitter, each time a frequency change would be required in the transmitter output frequency, the modulation source maximum frequency deviation would also require adjustment, making the modulation source maximum frequency deviation inversely proportional to the transmitter frequency. This necessarily would result in a great complication of the modulator circuitry, with corresponding transmitter cost increase. For the case of a frequency-shift-keyed transmission system, this is especially important, since very narrow bandwidth receiving band-pass filters would normally be used to filter the FSK modulation, for best interfering-signal rejection. Further, in the case of a frequency-hopping anti-jamming frequency system, the speed at which the transmission frequency could be changed may potentially be reduced, due to the potential necessity of additional time for the modulator maximum frequency deviation circuitry to stabilize after each adjustment, depending on the exact method of implementing the circuitry.